The invention generally relates to a method of making a semiconductor device, and more particularly, to a method of making a semiconductor nonvolatile memory cell.
Devices made from semiconductor materials are used to create memory circuits in electrical components and systems. Memory circuits are the backbone of such devices as data and instruction sets are stored therein. Maximizing the number of memory elements per unit area on such circuits minimizes their cost and thus is a primary motivation in the designing of such circuits.
FIG. 1 illustrates an exemplary prior art memory cell 20 which includes a vertically oriented, cylindrical pillar shaped junction diode 22 as the steering element of the cell and a storage element 24, such as an antifuse dielectric or a metal oxide resistivity switching layer. The diode 22 and the storage element 24 are interposed between top 26 and bottom 28 conductors or electrodes. The vertically oriented junction diode 22 includes a heavily doped semiconductor region 30 of a first conductivity type (such as n-type), an intermediate region 32 which is undoped semiconductor material or lightly doped semiconductor material (which will be referred to as an intrinsic region), and a heavily doped semiconductor region 34 of the second conductivity type (such as p-type) to form a p-i-n diode. If desired, the location of the p and n-type regions may be reversed. The semiconductor material of the junction diode 22 is generally silicon, germanium, or an alloy of silicon and/or germanium. Other semiconductor materials may also be used. The junction diode 22 and the storage element 24 are arranged in series between the bottom conductor 28 and the top conductor 26, which may be formed of a metal, such as tungsten and/or TiN. The storage element 24 may be located above or below the diode 22. Referring to FIG. 1A, U.S. Pat. No. 6,952,030, issued to Herner et al. and entitled “High-Density Three-Dimensional Memory Cell,” hereinafter the “'030 patent” and hereby incorporated by reference in its entirety, discloses an exemplary nonvolatile memory cell.
The resistance of metal oxide switchable resistors can be too low to be efficiently sensed by a three dimensional (3D) diode array. A low reset current is generally preferable to a high reset current, and thus the resistance of the resistor element is generally preferred to be high. Because the metal oxide element 24 is disposed over a diode pillar 22, the resistance of the oxide element can be too low, thereby giving rise to an undesirably high reset current. Furthermore, the metal oxide resistor material can be damaged as a result of etching during fabrication and thus fails to provide switching function.